In mining and construction environments, heavy-duty off-highway trucks are used to haul a variety of materials such as, for example, coal, rock, ore, overburden materials. Some environments expose the trucks to severe hauling conditions in which the material being hauled is relatively dense (e.g., 2700+ lbs./cubic yard) such as heavy metal ores (e.g., iron, gold and copper ore) and oil sands such as those mined in northern Canada. These types of materials can be extremely abrasive and cause severe wear on the inside of the bodies, particularly the floor area and particularly on the rear portion of the floor were the most material flows over the floor, thereby causing the most extreme floor wear. Moreover, material like the oil sands tend to be extremely cohesive and dump as one homogenous mass or glob of material (e.g., often called a “loaf” in the industry) that does not flake off or otherwise break apart and tumble as it is dumped. As used herein, the phrase “abrasive material” is material often high in silica and or material tending to be angular in composition and containing components such as silica having a hardness (e.g., MOHS value, Vickers Pyramid Number) greater than that of the material comprising the body hauling the material.
Typically, the sidewalls of a truck body are parallel to one another. For extremely abrasive materials, the bodies are sometimes made so the sidewalls are not parallel and instead taper away from one another from the front to the back of the body. By having the body sidewalls farther apart at the back of the body than at the front, as material is dumped from the body the material is relieved outwardly by the widening distance between the sidewalls as the material flows from the body. Looking from the rear of the body, the body sidewalls appear to funnel inwardly toward the front.
Bodies exposed to these severe applications are found primarily on very large capacity off-highway, haulage trucks such as those that have a payload of about 100 tons up to 400 tons and more. These trucks are typically equipped with open ended bodies—i.e., without tailgates. The rear of the truck body is open for easy material discharge and to ease the body loading process with loading shovel bucket's swinging in through the rear of the truck body.
As material is dumped from a truck body, it flows from the front of the body toward the back. As a natural result of this flow, the rearward portion of the body experiences more material moving over its surface than does the forward portion of the body and thus the most wear from any abrasion caused by the material flow. For hauling and dumping less abrasive material such as coal, the length of the hauling vehicle's body is not a serious problem with respect to the extra wear experienced toward the rear of the body's floor compared to the wear at the forward portion of the body. However, for material that is relatively highly abrasive, the disproportionate wearing of the rearward portion of the body is extreme and creates on going maintenance problems that significantly shorten the useful life of the body. And if, the body does not have a tailgate, its length is proportionally longer than necessary for statically supporting the load. This body length is necessary to carry the required load and to minimize the amount of material spilling out the open ended rear of the body as the truck travels along a haulage road. Such haulage roads are all part of a mine or construction project and are on private property and unsuitable for any public transport or transit.
Spillage of material on these privately operated haulage roads can be extremely costly. Spilt material often has sharp edges. Damage to the tires of the trucks traveling on the haulage route is likely to occur as the trucks drive over such spilled material. The spilled material may cut into the tires, substantially diminishing the tire life. Tires for large off-highway truck haulage vehicles often cost in excess of $20,000 to $40,000 per tire. As such, the tires of the vehicles represent a substantial investment that must be properly maintained and protected for maximum tire life. Good tire life on off-highway truck haulage vehicles approaches 8,000 and sometimes as much as 10,000 operating hours. In operations where there are large amounts of spilled material on the haulage roads, the operating hours per tire may drop as low as 1,000 to 2,000 hours per tire, which dramatically increases material haulage costs.
For trucks bodies without tailgates, the floor is typically angled upwardly at the rear of the body in order to assist in retaining the loaded material in the body (i.e., often a “V” shaped profile of the body when viewed from the side). In some bodies the loaded material retaining ability of the body is enhanced by “duck tailing” the rear portion of the body, which means that the body floor angle changes to a higher inclination angle at the rearward portion of the body in order to make it more difficult for material to spill as it is transported. Typically a “duck tailed” body has the last ⅓ to ⅕ portion of the body floor at such an angle to improve the body's ability load retention.
In low density material applications, these open-ended bodies are sometimes fitted with tailgates as a means to increase their volumetric loading capacity. For low density mined material such as coal, tailgates allow these very large trucks to carry greater volumes than otherwise possible without the tailgates. And to further increase the volume, the inclination of the rear of the floor is lowered. The higher volume of material does not overload the truck because it is of relatively low density such that the total weight of the load is within the limits specified for the truck chassis. An example of a body for an off-highway haulage vehicle with a tailgate is shown in applicant's U.S. Pat. No. 4,678,235, which is hereby incorporated by reference in its entirety for everything it describes. The tailgate does tend to raise the vertical elevation of the rear edge of the body over which a shovel bucket must clear the rear of the body as it swings a loaded bucket into the body for dumping into the body.